Bacterial pathogens usually fall in one of two groups: Gram-positive or Gram-negative. Antibacterial agents (including antibiotics) often exhibit selective activity for either Gram-positive or Gram-negative pathogens. Antibacterial agents that target both classes of pathogens are regarded as having broad spectrum activity.
There are many known classes of antibacterial agents such as the penicillins and cephalosporins, tetracyclines, sulfonamides, monobactams, fluoroquinolones and quinolones, glycopeptides, aminoglycosides, polymixins, macrolides, lincosamides, trimethoprim and chloramphenicol. The mechanisms of action of these various classes of antibacterial agents vary.
Resistant strains have evolved/arisen among Gram-positive pathogens such as Staphylococci, Streptococci, Mycobacteria and Enterococci, making the eradication of these strains very difficult. Examples of such strains include methicillin-resistant Staphylococcus aureus (MRSA), methicillin-resistant coagulase negative Staphylococci (MRCNS), penicillin-resistant Streptococcus pneumoniae and multiply-resistant Enterococcus faecium. 
Resistance to aminoglycosides, β-lactams (penicillins and cephalosporins) and chloramphenicol analogs is often expressed in pathogenic bacteria. This type of resistance is due to the bacteria-mediated modification of the antibacterial agent through either cleavage of the drug (as in the case with β-lactams) or formation of inactive derivatives (as in the case with aminoglycosides). As for the β-lactams, the resistance observed in clinical isolates is most commonly a result of the expression of “penicillinase” (a β-lactamase) that hydrolytically cleaves the β-lactam ring, thereby inactivating the antibacterial agent.
A more recent threat is the emergence of vancomycin-resistant (VRE) strains of enterococci (Woodford N., 1998, J. Medical Microbiology, 47(10):849-62). VRE strains are frequent causes of hospital-acquired infections and are unfortunately inherently resistant to most antibiotics. Vancomycin inhibits bacterial cell wall synthesis by binding to the terminal D-Ala-D-Ala residues of the cell wall peptidoglycan precursor. The high level vancomycin resistance of VRE isolates is termed VanA and is mediated by genes located on a transposable element which changes the terminal D-Ala-D-Ala residues to D-Ala-D-lac, thereby reducing the affinity for vancomycin.
Capreomycin is a cyclic homopentapeptide obtained from fermentation of Streptomyces caprolus (Herr, E. B., Jr., et al., 1960, Proc. Ind. Acad. Sci., 69:134) and is produced as a four-component mixture, with capreomycin IA and IB present as major products, and IIA and IIB as minor ones. Capreomycin has potent activity against mycobacteria, with little activity against other genera of bacteria.
                Capreomycin IA: R1═OH, R2=β-(S)-lysine amide        Capreomycin IB: R1═H, R2=β-(S)-lysine amide        Capreomycin IIA: R1═OH, R2═NH2         Capreomycin IIB: R1═H, R2═NH2         
Capreomycin itself is used clinically as a second-line treatment for tuberculosis but is not efficacious against most Gram-positive bacteria (as in the case with Staphylococcus) or Gram-negative bacteria (as in the case with Escherichia coli). Certain alkyl-, cycloalkyl- and halogen-substituted phenylurea analogs of capreomycin have been demonstrated to be broad-spectrum (Gram-negative and Gram-positive) antibacterials, especially against resistant strains (Dirlam, et al., Bioorganic and Medicinal Chemistry Letters, 1997, 7(9), 1149-1152).
In light of the rapid emergence of multidrug-resistant bacterial pathogens, the development of antibacterial agents that are effective against both Gram-positive and Gram-negatives pathogens, irrespective of their resistance profiles, and particularly against VRE and MRSA, is urgently needed.